Typically, in the cast molding of contact lenses, a single mold assembly for molding a single contact lens product includes a male mold section having a convex optical surface defining a posterior surface of the lens to be made, and a female mold section having a concave optical surface defining an anterior surface of a lens to be made. Thus, when the male and female mold sections are assembled together, a contact lens shaped cavity is formed between the concave surface of the female section and the convex surface of the male section.
A conventional apparatus for dispensing a lens precursor composition, for example, a polymerizable composition, is based on a syringe pump and commonly dispenses from about 30 microliters to about 80 microliters of polymerizable composition in a single mass onto the concave surface of the mold section. Due to variability in the volume of composition dispensed by such syringe pumps, the amount of composition dispensed onto the mold section typically exceeds the amount needed to produce the lens in order to avoid having less than the amount of composition necessary to form a lens present in the mold.
For example, such syringe pump systems have optimal rates of filling and emptying which are relatively slow, as operating at faster rates can cause air bubbles to form in the composition and/or can result in higher pressures to be present in the barrel, causing the composition to continue to flow out after the plunger has stopped moving. Additionally, it is common for a small amount of the dispensed composition to cling to the outside of the dispensing needle, resulting in the volume of the composition deposited into the mold being lower than intended. When another portion of the composition is subsequently dispensed through the needle, some of the composition already clinging to the outside of the needle may be dispensed along with the portion of the composition which is intended to be delivered, resulting in the volume of composition actually deposited into the mold being larger than intended. Furthermore, as the viscosity of the liquid typically is related to the temperature, the volume of composition delivered can vary over time due to changes in temperature, and can vary from syringe to syringe when a series of syringes are used, as is commonly done in a manufacturing setting. Due to these factors, as well as others, when using a syringe pump to dispense same size portions of composition into a plurality of contact lens molds, the actual volume of composition deposited into the molds can vary considerably, for example, by 5% or 10% or more, and it is difficult to predict and compensate for the variability.
Dispensing an excess volume of the composition into the mold results in the excess composition being “squeezed” out of the lens shaped cavity formed when the male mold section is combined with the female mold section. This excess composition may end up as a ring or partial ring of polymerized material outside the lens-shaped cavity is referred to as flash. The composition which ends up as flash is wasted since it cannot be reused to produce other lenses. Such waste of composition increases the cost of manufacturing cast molded lenses, particularly if the composition is relatively expensive, such as is typically true of compositions used to produce silicone hydrogels. The presence of flash can further increase manufacturing costs as additional process steps may be required to ensure that the flash is captured and removed during the manufacturing process.
Further, using the method of filling the female mold section in a single mass does not allow for pre-coating the mold, and can result in undesired mixing of the different compositions used and/or an uneven or non-uniform placement of a relatively thin layer of a coating or treatment composition over or on top of the surface of the basic lens composition, for example, polymerizable composition.
Thus, there is a need for new methods for manufacturing contact lenses, new apparatus useful in producing contact lenses and for new contact lenses manufactured/produced using such methods and/or apparatus.